1. Field of the Invention
The invention relates to paving machines and, more particularly, relates to an improved method and apparatus for uniformly heating a screed plate of a paving machine by providing a conductive plate between an electrical heating element and the screed plate, and for providing a clamping mechanism that permits the electrical heating element to be easily replaced without the need to remove the screed plate.
2. Discussion of the Related Art
Paving machines are well known for working paving materials into a mat to produce roads and other paved structures. Specifically, the typical paving machine transports paving materials from a hopper along a conveyor system and ultimately to a distributing auger, where the paving materials are distributed onto a roadway or another surface, where a screed plate then paves the paving materials into a mat. While the paving materials could be any of various known materials, hot mix asphalt (HMA) is commonly used and, for the sake of convenience, the paving materials will hereinafter be referred to as HMA.
The screed plates of HMA paving machines are typically preheated to a temperature of about 200.degree. F. to 300.degree. F. before paving commences and are maintained at this temperature during paving to prevent the hot asphalt being leveled by the screed plate from congealing on the face of the screed plate. Screed plates have traditionally been heated by oil or gas burners mounted above the screed plate such that the flames from the burners impinge sheet metal plates on top of the screed plate. Such burners supply intense heat to localized portions of the screed plates which results in uneven heating and congealing of the HMA onto the screed plate. Additionally, if the process is not carefully controlled, the screed plate may warp and become ineffective. Furthermore, as the flames become progressively dirtier, noxious fumes are emitted for the operator to contend with.
Systems have been proposed which are designed to avoid or to at least alleviate some of the problems associated with traditional screed heaters. In one such system, a heater heats the screed plate of a paving machine via heat transfer from heating oil stored in a low pressure reservoir mounted directly on top of the screed plate. Oil in the reservoir is drawn from the reservoir, pressurized by a high pressure pump, and then fed through a pressure release valve or other suitable flow restrictor which creates a pressure drop in the range of about 700 to 800 psi, thereby heating the oil to a temperature of about 275.degree. F. The thus-heated oil is then returned to the reservoir for heat transfer to the screed plate.
This heated oil system suffers from several drawbacks and disadvantages. Most notably, the large pressure drops needed to provide the necessary heating require that the heating oil be pressurized by a pump to a relatively high pressure in the range of 800 to 1000 psi before undergoing the pressure drop in the flow restrictor. This requires the use of high pressure hoses and connections throughout the system, thus increasing the cost and complexity of the system and also increasing the dangers of leaks which could render the system ineffective. Moreover, if for any reason the pump and relief valve are not capable of providing a sufficiently large pressure drop to adequately heat the oil, the system then becomes incapable of boosting the oil temperature to the required level.
It therefore became desirable to develop a screed plate heating system that involves no moving parts, runs clean, emits no noxious fumes, and is capable of uniformly heating the screed plate.
One known system that strives to meet at least some of these goals involves the installation of electrical heating elements that are in direct contact with the screed plate to heat the screed plate. Being electrically powered by a sufficiently sized generator, this system does not have the disadvantages associated with combustion, and also ensures that sufficient energy is supplied to the electrical heating elements so that the screed is adequately heated. Furthermore, the generators associated with the electrically heated systems allow the use of higher-wattage lights than the conventional twelve-volt lights used on traditional paving machines, thus facilitating night operation. However, the direct contact between the heating elements and the screed plate gives rise to heat distribution problems similar to those encountered by oil-heated screeds. Specifically, hot spots develop on the screed plate at the point where the heating element contacts the screed plate, and the screed plate cools progressively at points more distant from the contact. This uneven heat distribution can also lead to relatively high temperature gradients, and possible warping of the screed plate. Another disadvantage arises when the electrical heating elements require either repair or replacement. In order to remove a heating element from this system, the screed plate must first be removed before an operator is able to access the heating element. This removal requirement is very time consuming and labor intensive.
The need has therefore arisen to provide an electrically heated screed assembly, which is capable of uniformly heating the screed plate while allowing easy access to and replacement of the heating elements without having to remove the screed plate.